The enhancement of pain following injury occurs acutely within a few minutes, and can last for several days or weeks. In some cases, the enhance pain can last indefinitely, either because the injured cannot be repaired, or because of some unknown mechanism that may not be related to peripheral injury. In these cases, the pain is defined as "chronic pain." It has been proposed that long-term exposure to painful stimuli can cause profound changes in the central nervous system that are very difficult to alter and are the cause of chronic pain. Chronic pain has proven to be extremely debilitating for affected individuals, and has been refractory to most treatments. We hypothesize that one of the CNS locations in which pain transmission is altered by persistent nociceptive inputs is the spinal cord. Just as a number of peripheral events contribute to short-term sensitization of peripheral nociceptors (see Projects 2 and 3), it is likely that long-term spinal cord sensitization of peripheral nociceptors (see Projects 2 and 3), it is likely that long-term spinal cord sensitization requires a combination of factors involving damaged peripheral tissue, afferent traffic from that tissue, and spinal responses to the peripheral injury. We propose that understanding, long-term spinal sensitization requires systematic manipulation and evaluation of combinations of peripheral, afferent and central effects that have been implicated individually as contributors to persistent pain. Once these factors are understood, rational treatments can be developed. The specific aims for the next 5 years are to determine: 1. the contribution of primary afferent firing in our present model of long- term hyperalgesia. 2. whether blockade of afferent activity, axonal transport of peripheral nerve ending destruction a) affects the glial activation and allodynia/hyperalgesia produced by peripheral injection of formalin, and/or b) produces glial activation and allodynia/hyperalgesia without peripheral tissue damage. 3 whether activation of spinal cord glial cells (astrocytes and microglial cells) is involved in short- and long-term hyperalgesia. 4. some of the potential mediators that activate astrocytes and microglia and induce long-term hyperalgesia in our formalin model. Combinations of single afferent recording, behavior, and immunohistochemistry will be used on rats and transgenic mice to accomplish these aims.